The invention lies in the printing technology field and relates, more specifically, to a method of modifying the wetting properties of a printing form having a semiconducting surface, and a printing form having a semiconducting surface which has different wetting properties. The invention further pertains to the use of the novel concept in an offset printing process.
European patent EP 262 475 B1 describes a printing press which is equipped with a printing form on which an image to be printed can be represented by corresponding hydrophobic and hydrophilic regions. To permit switching between the hydrophilic and hydrophobic state in the individual regions, ferroelectric material which is locally polarizable and depolarizable is provided on the printing form. The hydrophilization or rehydrophilization of the printing form is accordingly effected by a polarization or depolarization mechanism which can be carried out reversibly within the printing press. However, the disadvantage of this process is that the effect is based on long-range electrostatic attractive forces and accordingly the resolution of the image to be printed is limited by the long-range electrical attractive forces.
U.S. Pat. No. 3,678,852 discloses a printing form which is coated with an amorphous semiconductor. The amorphous state of the semiconductor can be changed with the aid of a laser beam from the disordered amorphous state to a more highly ordered crystalline state. In the crystalline state, the semiconducting surface is rougher so that the reordering of the semiconducting surface results in liquids adhering better in the region of the rougher surface than in the amorphous smooth regions. The resolution of the printing form produced according to that process is limited by the minimum size of the crystalline regions.
It is accordingly an object of the invention to provide a printing form and a method of defining its wetting characteristics, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for an alternative process for locally and repeatedly modifying the wetting properties of a printing form having a semiconducting surface and to propose a corresponding printing form.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of modifying a wetting characteristic of a printing form, such as a printing plate or a printing cylinder, with a semiconducting surface. The novel method comprises the following steps:
bringing a semiconducting surface of a printing form into a first chemical state having a first wetting property; and
bringing a portion of the semiconducting surface into a second chemical state having a second wetting property different from the first wetting property.
In accordance with an added feature of the invention, the second chemical state is set by modifying chemical terminal groups of the semiconducting surface and/or modifying a chemical property of first atomic layers in a surface region of the semiconductor having the semiconducting surface.
The basic concept of the present invention is found in the modification of the local wetting behavior, i.e. the local hydrophilic or hydrophobic behavior of a printing form, by controlling the chemical terminal groups of the surface with correspondingly different electronic properties, i.e. interaction properties. For this purpose, a surface having a chemical structure which has a preferably essentially uniform hydrophilic or hydrophobic wetting property is first produced. This surface is then converted in localized areas by localized modification of the chemical structure into the respective other state of the wetting property, i.e. from hydrophilic to hydrophobic or from hydrophobic to hydrophilic. In the case of this chemical switching process, it is not necessary to use special ferromagnetic materials or to produce a modification of the surface roughness, for example by crystallization. Rather, the wetting behavior in the individual regions of the semiconducting surface is controlled by providing the semiconducting surface in a controlled manner with hydrophilic and hydrophobic chemical terminal groups.
This localized switching process can be effected, for example, with the aid of so-called chemical processing, in which the chemical conversion is effected by photothermal or photochemical, or generally by means of laser-induced, reaction processes.
In accordance with an additional feature of the invention, the first chemical state is produced by layer removal at the surface of the semiconductor in an atomic dimension. The layer removal may be carried out with HF or an ammonium fluoride solution.
In accordance with another feature of the invention, the second chemical state may be produced by localized chemical processing in regions of the semiconducting surface.
In accordance with again an added feature of the invention, the surface is processed with a controlled energy source and the energy source is controlled such that the second chemical state is produced to correspond to image information to be printed with the printing form or to its negative.
In a preferred embodiment of the invention, the controlled energy source is a laser, for instance a pulsed laser, or a source of non-coherent energy (i.e., a conventional energy source), such as a UV lamp.
In accordance with again another feature of the invention, the printing form surface is processed with a fluorine laser having a VUV wavelength of 157 nm, an excimer laser having a UV wavelength xe2x89xa6308 nm, or a solid-state laser having a wavelength xe2x89xa6355 nm.
A particularly suitable energy source is an Nd:YAG laser.
In a preferred embodiment, silicon is chosen as the semiconductor. This semiconducting surface is first converted into a hydrophobic state, with, for example, SiH, SiH2 and/or SiH3 groups being introduced into the surface or applied to the surface. In order to change the hydrophobic behavior, the hydrophobic group is then exchanged locally for a hydrophilic group or is converted into such a group so that, for example, SiOH, SiOSi and/or SiO units replace the hydrophobic groups.
When a silicon face is used as the surface of the printing form, there is a particular advantage that the surface is atomically smooth and the hydrophilic or hydrophobic terminal groups can be applied essentially the same distances apart.
Various processes are suitable for producing a hydrophilic or hydrophobic starting layer and for the switching process between hydrophilic and hydrophobic.
Thus, for example for the production of a uniform hydrophilic surface, the printing form can be subjected to a suitable wet chemical modification process, with the result that, under suitable conditions, it is possible to produce highly hydrophilic wettability of the surface which is caused, for example, by the incorporation of SiOH and/or SiO groups into the first atomic layers of the semiconducting surface. By exposure to a laser of suitable wavelength, in particular to a pulsed laser, this hydrophilic wetting property can now be changed specifically and locally into a hydrophobic wetting property by replacing the hydrophilizing group by a hydrophobic surface configuration.
However, a process taking place in the opposite direction is also possible. Here, an essentially hydrophobic surface of the printing form is first produced. For this purpose, the printing form can, for example, be treated with a dilute HF solution or an ammonium fluoride solution, only the uppermost layers of the semiconductor being removed and a hydrophobic, hydrogen-terminated surface forming. This can then be hydrophilized again in individual regions, energy being supplied locally to these regions.
After the use of the printing form, i.e. after printing, the total surface can be converted back into the starting state.
The printing form is then available before being provided with further images.
With the method according to the invention, it is possible to provide a printing form which can be repeatedly provided with images and can thus be reused in many successive cycles. In addition, the resolution of the printing form is not limited by the size of the crystals or an electrical interaction.
With the above and other objects in view there is also provided, in accordance with the invention, a printing form, comprising:
a semiconducting surface carrying a pattern of hydrophilic regions and hydrophobic regions;
the hydrophilic regions having a first chemical state and the hydrophobic regions having a second chemical state differing from the second chemical state.
As noted above, the semiconducting surface is formed on a printing plate or on a printing cylinder or printing roller.
In accordance with again a further feature of the invention, the semiconducting surface is formed by silicon, germanium, or an alloy of silicon and germanium. The phase of the semiconductor may be amorphous, polycrystalline, and/or crystalline.
Specific semiconducting material are SiGe, SiC, and SiCN.
In accordance with a concomitant feature of the invention, the second chemical state extends to a thickness of no more than 5 nm from the semiconductor surface into a semiconductor material defining the semiconductor surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a printing form and method of modifying the wetting characteristics of the printing form, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.